Einfluss von Reoxidationszyklen auf die Betriebsfestigkeit von anodengestützten Festoxid-Brennstoffzellen

Ettler, Manuel

Dissertation / PhD Thesis/Book

Einfluss von Reoxidationszyklen auf die Betriebsfestigkeit von anodengestützten Festoxid-Brennstoffzellen

Ettler, M. (Corresponding author)FZJ*

2009
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-570-8

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment 36, 138 S. (2009) = Bochum, Univ., Diss. 2008

Please use a persistent id in citations: http://hdl.handle.net/2128/3614

Abstract: Fuel cells are electrochemical devices which directly convert the chemical energy of a fuel into electrical energy without conversion losses to other forms of energy. The various types of fuel cells can be classified by their electrolyte material. In the case of a solid oxide fuel cell (SOFC), this material is an oxygen-ion-conducting oxide ceramic, mostly yttria-stabilized zirconia (YSZ). Forschungszentrum Jülich has been developing the SOFC concept of a planar cell design based on an anode substrate for approx. fifteen years. In this concept, the anode of the electrochemical cell consists of two parts: the mechanically supporting, thicker and more porous anode substrate, and a thinner, fine-structured so-called anode functional layer. Both layers are made of cermets (composite of a ceramic material in a metallic matrix) of nickel and YSZ. The anode substrate is fabricated by warm pressing a powder mixture made by the Coat-Mix$^{®}$-process or by tape casting and is then coated with the anode functional layer, as well as electrolyte and cathode by vacuum slip casting or screen printing. Both substrate and functional layer are produced using powder mixtures of NiO and YSZ, so that the sinter process can be performed in air. The NiO is reduced to nickel during the initial operation of the cell or stack. A fundamental limitation of the reliability of the fuel cell is an intended (by air supply) or accidental (by system failure) increase in the oxygen partial pressure on the anode side of the cell. This increase may cause reoxidation of the nickel in the anode, leading to irreversible structural changes in the microstructure and to the macroscopic expansion of both the anode substrate and the anode functional layer. The expansion induces tensile stresses in the electrolyte. If these stresses exceed its residual stresses and its tensile strength, cracks will occur in the electrolyte layer leading to complete cell failure. The aim of this work is the characterization of cells based on various types of substrates with respect to their behaviour upon reoxidation of the nickel in the anode. Tests were therefore carried out on so-called free-standing half cells consisting of anode substrate, anode functional layer and electrolyte. In various series of measurements, the influences of temperature, time of reoxidation, air flow to the anode, substrate porosity and substrate thickness were investigated on the mechanical behaviour of the cell upon reoxidation. The experiments showed that the observed parameters strongly affect the mechanical behaviour of the cell in terms of degree and homogeneity of oxidation. The dominant processes for the progress of reoxidation were identified and the progress of the reoxidation process was described in terms of a model. The investigation of the mechanical integrity of the electrolyte after each test permitted the determination of the maximal tolerable degree of oxidation causing no mechanical damage that permanently affected the integrity of the fuel cell. Moreover, so-called half-cassettes (half cells assembled to a metal frame) were reoxidized and reduced repeatedly (redox cycled) to study the effect of the redox cycling of cells in a fixed state and therefore in a system-like configuration. The investigations revealed information for the definition of specifications regarding temperature, time of reoxidation and air flow for the possible intended redox cycling of stacks which were taken into account for system operation. The studies provided important data regarding the operational stability of SOFC systems operated as auxiliary power units (APUs) which were not available prior to the underlying research project. These data were fed directly into the development of other system components (e.g. the reformer), as well as the definition for possible modes of operation for SOFC-APU systems. Finally, it was successfully demonstrated that the durability of fuel cells can be improved by optimizing the specifications for redox cycles.

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